Overhauser DNP

Although O-DNP was originally discovered in alkali metals, its main application today is in DNP in solution, predominantly for and for C NMR, although many other spin 1/2 nuclei can be polarized. Earlier applications also included and C DNP of organic conductors [46 8]. 

The O-DNP experiment requires irradiation at the electron Larmor frequency to saturate the electron transition. The enhancement arises from subsequent relaxation processes involving simultaneous reversals of I and S in opposite directions (flip-flop transitions, Wq), or in the same direction (flip-flip transitions, IT2). This is depicted by the energy level diagram in Fig. 3. Hausser and Stehlik introduced a phenomenological description using rate equations [37], based on Solomon s treatment of the OE. According to the Solomon equations [49], the rate equation for the expectation value of the nuclear polarization /z can be written as  

Top) The noisy Li resonance. Middle) The Li resonance enhanced by electron saturation. Bottom) Proton resonance in glycerin sample. (Copyright 1953 by The American Physical Society) 

S represents the electron and I a spin 1/2 nucleus. Transitions are labelled with the respective transition probabilities ITis and Wn are the transition probabilities for the electron S and for the nucleus /, respectively. Wo and W2 represent the zero and double quantum transition probabilities, respectively 

The coupling factor describes the contribution of the transition probabilities Wj towards the maximum polarization and depends on the nature of the short range interactions that determine these transition probabilities. 

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The Overhauser effect has been widely employed as an NMR analysis method in many disciplines ranging from medical to chemical sciences, and broadly refers to the motion-mediated transfer of spin polarization from a species with a higher gyromagnetic ratio (y) to one with a lower gyromagnetic ratio. Because molecular motion is critical for efficient transfer, the Overhauser effect is most commonly observed in liquid samples. The Overhauser effect can be divided into two categories the nuclear Overhauser effect (NOE), where both species are nuclear spins, and Overhauser DNP, where the higher y spin is an unpaired electron. As Overhauser DNP is the focus of this review, some of the terminology and equations are specific to the Overhauser DNP effect. 

The mathematical formulation for Overhauser DNP comes from the differential equation for population changes of the nuclear spin state J, from the initial value J0 to the excited value (Jz ), originally described by Solomon28 ... 

While these models match experimental data reasonably well at lower fields, recent experiments at higher magnetic fields of 3.4 and 9.2 T show enhancement values that are much higher than predicted with the currently employed theory.41,72,79 At these higher fields, the timescale of molecular interactions that give rise to Overhauser DNP effects is much shorter (sub-picoseconds to picoseconds) and thus should be more sensitive to the rotational diffusion dynamics of water, closely related to the atomistic details of the radical and solvent, instead of translational diffusion dynamics. These atomistic details are not accurately represented in the FFHS or rotational models (Equations (13) and (15)), implying that further work needs to be done to develop more accurate models. 

This X-band Overhauser DNP technique has also been developed to study proteins and their interactions with site-specific resolution, employing site-directed mutagenesis and spin-labelling methods128 to... 

PEDRI (also known as Overhauser MRI) integrates the principles of Overhauser DNP with MRI to provide information on the spatial distribution of free radicals within the system of interest. Much work has been done to... 

The choice of the polarization mechanism depends entirely on the experimental conditions. Overhauser DNP is the only DNP mechanism known to work in liquids. The specific mechanism in the solid state depends primarily on the experimental conditions, i.e., choice and concentration of the radical. Table 1 summarizes the DNP mechanisms and the conditions required for these mechanisms. The key mechanisms will be reviewed in the following paragraphs. 

Overhauser DNP (O-DNP), driven by the Overhauser effect (OE), where the entire process is carried out in the liquid state. 

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